That's on top of the 1.7 million-square-foot headquarters complex Vulcan already is building for Amazon a few blocks to the north.

Amazon is scheduled to start moving into the first of those buildings next month, Vulcan spokeswoman Lori Mason Curran said. It will begin moving into 2201 Westlake by midyear, she added.

Amazon, which sells everything from books to bikes, has continued to grow despite a global economic downturn. Its Web site lists more than 900 job openings in Seattle, many for its Kindle e-reader business.

Spokeswoman Mary Osako said in an e-mail that 2201 Westlake, at Westlake Avenue and Denny Way, "will help us to accommodate our growing employee base."

When rumors of Amazon's interest in additional space first surfaced several months ago, brokers said they understood the company wanted something short-term. But Osako said 2201 Westlake "will be part of our new South Lake Union headquarters on a long-term basis."

Neither she nor Mason Curran would disclose the lease's term. Amazon has said its leases for the other buildings in the headquarters complex are for up to 16 years, with options to renew for up to 10 more.

The new deal means the office space at 2201 Westlake is fully leased, making it the first of the recently completed office towers in greater downtown to reach that milestone. Another half-dozen buildings, which like 2201 were started without signed tenants before the market collapsed, remain mostly or entirely empty.

At 2201, global health nonprofit PATH already has moved into the remaining 113,000 square feet in the 12-story building.

"Vulcan is validating the vision they had for South Lake Union," said Kip Spencer, co-founder of the online commercial real-estate database Officespace.com. "That neighborhood is going to be very impressive once all these new companies move in."

Callahan agreed. "So far they're hitting it out of the ballpark with their South Lake Union investments," he said of Vulcan.

Amazon's Osako said employees moving to 2201 would come from several other Seattle work sites. The company now leases space in several buildings in and near downtown, including its present headquarters at the Pacific Medical Center on Beacon Hill.

Those leases are scheduled to expire by the end of next year. Amazon won't say exactly how many people it employs in Seattle, only that there are several thousand.

Amazon made a 2009 profit of $902 million, up 40 percent from 2008, as sales topped $24 billion. It ended the year with $3.4 billion in cash on hand, causing some speculation that it's on the hunt for businesses to buy.

"E-commerce in general probably is going to double in the next five years, and Amazon is the leader," said Colin Gillis, an analyst who follows Amazon for BGC Financial in New York.

The company bought Internet shoe and apparel retailer Zappos.com last year in a cash-and-stock transaction that Amazon valued at about $807 million. Gillis said Amazon is in an "acquisitive mode," possibly explaining at least part of its South Lake Union expansion.

"They have the balance sheet to do small and large acquisitions," he said. "But their organic growth alone is healthy."

In addition to its present space commitments in South Lake Union, Amazon has options to lease another 500,000 square feet from Vulcan.